1. Field of the Invention
This invention relates to magnetic thin film heads (TFH) for recording and reading magnetic transitions on a moving magnetic medium.
2. Background of the Invention
Magnetic TFH transducers are known in the prior-art. See, e.g. U.S. Pat. Nos. 4,016,601; 4,190,872; 4,652,954; 4,791,719 for inductive devices and U.S. Pat. Nos. 4,190,871 and 4,315,291 for magnetoresistive (MR) devices.
In the operation of a typical inductive TFH device, a moving magnetic storage medium is placed near the exposed pole-tips of the TFH transducer. During the read operation, the changing magnetic flux of the moving storage medium induces changing magnetic flux upon the pole-tips and gap between them. The magnetic flux is carried through the pole-tips and back-portion core around spiralling conductor coil winding turns located between the core arms. The changing magnetic flux induces an electrical voltage across the conductor coil. The electrical voltage is representative of the magnetic pattern stored on the moving magnetic storage medium. During the write operation, an electrical current is caused to flow through the conductor coil. The current in the coil induces a magnetic field across the gap between the pole-tips. A fringe field extends into the nearby moving magnetic storage medium, inducing (or writing) a magnetic domain (in the medium) in the same direction. Impressing current pulses of alternating polarity across the coil causes the writing of magnetic domains of alternating polarity in the storage medium. Magneto-resistive (MR) TFH devices can only operate in the read mode. The electrical resistance of an MR element varies with its magnetization orientation. Magnetic flux from the moving magnetic storage medium induces changes in this orientation. As a result, the resistance of the MR element to a sensing electric current changes accordingly. The varying voltage signal is representative of the magnetic pattern stored on the magnetic medium.
Prior-art magnetic recording inductive thin film heads include top and bottom magnetic core pole layers, usually of the alloy Nixe2x80x94Fe (permalloy), connected through a via in the back-portion area, and separated by a thin gap layer between the pole-tips in the front of the device. The bottom pole-tip is usually designed to be wider than the top pole-tip in order to prevent xe2x80x9cwraparoundxe2x80x9d due to misregistration or misalignment, as taught by R. E. Jones in U.S. Pat. No. 4,219,855. Alternatively, one or both pole-tips are trimmed by ion-milling or by reactive ion etching (RIE) to ensure similar width and proper alignment. Such a technique is disclosed, for example, by U. Cohen et al. in U.S. Pat. No. 5,141,623. As the track width decreases in order to increase the recording density, the write head pole-tips must be very narrow. P. K. Wang et al. describe elaborate schemes to obtain pole-tips for writing very narrow track width, in IEEE Transactions on Magnetics, Vol. 27, No. 6, pp. 4710-4712, November 1991.
One of the problems associated with the prior-art pole-tip designs is that during write operations, substantial noise is introduced along the track-edges (on the magnetic storage medium), which adds to the noise generated by the medium during read operations. During the write operations, significant portions of the intense magnetic flux lines, emanating from the corners and side-edges of the pole-tips, deviate from a direction parallel to the track""s length. The non-parallel magnetic field magnetizes the medium in the wrong directions, giving rise to noise along the track-edges. This noise is usually characterized as xe2x80x9ctrack-edge fringing noisexe2x80x9d and is a major obstacle to increasing the track density. According to a paper by J. L. Su and K. Ju in IEEE Transactions on Magnetics, Vol. 25, No. 5, pp 3384-3386, September 1989, the track-edge noise extends about 2.5 xcexcm on each side of the written track. As track density increases, the track width decreases along with the strength of the read-back signal. If the track-edge fringing noise remains the same, the signal to noise ratio (SNR) is directly proportional to the track width, and deteriorates rapidly as the latter decreases. The current state-of-the-art magnetic thin film media can support lineal density of about 40,000-60,000 flux changes per inch (FCI), corresponding to domain length of about 0.4-0.7 xcexcm. Yet, the track width is at least an order of magnitude larger, about 8-12 xcexcm. There is no apparent reason why the media could not support much narrower tracks, if not for the rapid deterioration of the SNR. By eliminating most of the track-edge fringing noise, the useful track width could be decreased to about 1.0 xcexcm, or less. This represents an increase of recording density by about an order of magnitude.
In addition to the medium""s noise, there is also the head""s noise. A significant portion of the head""s noise is due to edge-closure domains in the pole-tips. This noise contribution becomes more dominant as the width of the pole-tips decreases. This problem was described, for example, by D. A. Herman in Paper No. 299, xe2x80x9cLaminated Soft magnetic Materialsxe2x80x9d, The Electrochemical Society Confererice, Hollywood, Fla., October 1989.
The present invention provides an inductive xe2x80x9cpinched-gapxe2x80x9d thin film head (TFH) device having pole-tips that are in substantial contact along their side-edges, thereby enclosing a gap segment. Since no magnetic flux lines emanate from the corners and side-edges, the write magnetic field is precisely confined to across the pinched-gap segment in a direction parallel to the track""s length. As a result, the usual noise-producing non-parallel magnetic field from the pole-tips"" corners and side-edges is virtually eliminated. The written (medium) track width is precisely defined by the width of the pinched-gap segment. It incorporates a high degree of magnetization coherency and is substantially free of the track-edge noise.
An effective technique to confine the gap includes photolithographic definition and etching gap-vias through the gap to the bottom pole-tip sides or side-edges, followed by deposition of the top magnetic pole. The distance between the gap-vias defines the width of the pinched-gap segment, which in turn accurately and consistently determines the width of the written track. The total width of the pole-tips is not as crucial as in prior-art devices. Either one or both pole-tips may be deposited wider than their final dimension and, following the top pole deposition, trimmed by ion-milling or by reactive ion etching (RIE) to their final width. Depositing wider pole-tips improves the composition and thickness uniformities of the device.
Alternatively, intentional wraparound on both sides of the bottom pole-tip also produces a confined gap segment with substantial contact of the pole-tips along their side-edges. Depositing a top pole-tip that is wider than the bottom one readily produces such a wraparound. The width of the pinched-gap segment (as well as the track width) is determined by the width of the bottom pole-tip. Incomplete step coverage by the gap layer at the side-edges of the bottom pole-tip provides substantial contact there. To ensure contact along the upper corners of the bottom pole-tip, they can be exposed by ion-milling (with a thin mask) or by etching gap-vias prior to the deposition of the top pole. Excess width of the top pole-tip on both sides of the bottom pole-tip can be trimmed by ion-milling, RIE, or by chemical etching.
Although the pinched-gap transducer""s efficiency is not adversely affected in the write operation, it may be impaired in the read mode due to the partial shorting of the pole-tips. A dual-element or tandem TFH device, combining a pinched-gap TFH device as a write element, and a separate TFH device as a read element may be advantageous. The pinched-gap TFH device is particularly suitable for use as a write element in combination with a separate read element, such as a magnetoresistive (MR) element, or another inductive TFH element optimized for the read operation. Much of the head""s noise, such as Barkhausen pop-corn noise, or glitch after write, is related to the write operations. In a dual-element device combining the pinched-gap as the write element, such noise is irrelevant since it does not interfere with the read-back and verification operations. These functions are executed by the separate read element. Also, the pinched-gap TFH device exhibits less noise due to the elimination of the edge-closure domains in the pole-tips. This is particularly true when the pinched-gap device is used in the write mode, but it is also effective in the read mode. combining a pinched-gap write element with an MR read element is particularly advantageous, especially for high density small form disk drives. However, the fabrication of the additional MR element is highly complex and very costly A simpler and more economic solution is to fabricate a separate inductive TFH element, optimized for the read operation.
The read and write TFH elements can preferably be situated or placed side-by-side on the same rail (or air bearing surface) or on different rails of the slider. Such a layout requires a fixed-translation of the slider to position the read element over the written track after a write operation. However, such a short translation takes only a very short time. Alternatively, multiple sliders per disk surface, attached to separate actuators, can be positioned in such a way that the read element of the second slider follows directly behind the track of the first slider""s write element. Such a track-following scheme minimizes the time lapse between a write and a read-back or verification operations. These fixed-translation and track-following schemes can also apply for a dual element or tandem combination of an inductive write element and an MR read element.
The pinched-gap TFH device of this invention may also be used with contacting heads, such as in magnetic tape recording devices or contact recording disk drives. Such heads may be positioned in sliders having one or more sliding rails.
An object of this invention is to provide a pinched-gap TFH device for writing media tracks virtually free of track-edge noise.
Another object is to provide a pinched-gap TFH device having accurately and consistently defined narrow pinched-gap segment and capable of writing narrow tracks, down to xe2x89xa61 xcexcm.
An additional object is to decrease the head""s noise by reducing the edge-closure domains along the side-edges of the pole-tips.
Another object of the invention is to provide methods for making the pinched-gap TFH device.
A still further object of this invention is to provide a dual-element TFH device with a pinched-gap write element and an MR or inductive TFH read element.
An additional object is to provide a dual-element TFH device having the read and write elements placed side-by-side on the same rail (or air bearing surface) or on separate rails of the slider.
A further object of the invention is to provide a fixed-translation procedure for a read-back or verification of a written track, using the dual-element TFH device with side-by-side elements on the same rail or on separate rails of the slider.
Another object is to provide a track-following scheme for multiple actuators having more than one head per disk surface, to minimize the time lapse between the write and read-back or verification operations.